9.11 Predicate Specifiers

Some Gnus variables are predicate specifiers. This is a special
form that allows flexible specification of predicates without having
to type all that much.

These specifiers are lists consisting of functions, symbols and lists.

Here's an example:

(or gnus-article-unseen-p
gnus-article-unread-p)

The available symbols are or, and and not. The
functions all take one parameter.

Internally, Gnus calls gnus-make-predicate on these specifiers
to create a function that can be called. This input parameter to this
function will be passed along to all the functions in the predicate
specifier.